1Department of Anesthesiology and Pain Relief Center, The University of Tokyo Hospital, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan2Department of Rehabilitation Medicine, Graduate School of Medicine, The University of Tokyo, Tokyo, Japan3Kobe Advanced ICT Research Center, National Institute of Information and Communications Technology, Kobe 651-2492, Japan

Received 20 December 2010; Revised 3 March 2011; Accepted 25 March 2011